It's one of those brain things with the tune and singing being heard.
To stop it, concentrate on something else or listen to some different music.
As a bit of humour - you can move away from the singing men so they do not sing into your ear or plug your ears with cotton wool so that you do not hear them. :)
Seriously though, sometimes a song just gets stuck in a loop that you can't seem to get rid of it, and you need to distract yourself with something else.
If it is just buzzing, whooshing and whistling noises, this is called tinnitus and unfortunately there is no cure to get rid of it. Sometimes it can be caused by infections and congestion or a build up of wax pressing on the ear drum. This type of problem would need a doctor's examination to find out the cause.
If it sounds like there are actually people in the room (or standing in your ear) and singing, then you should get screened for things like schizophrenia. The exception is if it only happens when there's a lot of white noise around (like fans or refrigerators running)--the sound from things like these can get distorted into a musical hallucination, but you won't notice it in a quiet place. The occasional thing like a phantom cell phone ring isn't worrisome either.
If it's just stuck in your head, then you can try to distract yourself, listen to different music, or just wait it out.
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